The present invention relates generally to object tracking systems, and specifically to non-contact, electromagnetic medical systems and methods for tracking the position and orientation of an object. The present invention is also directed to a novel calibration method for electromagnetic-based medical tracking systems that can account for the effects of interference from nonmoving metallic objects.
Non-contact methods of determining the position of an object based on generating a magnetic field and measuring its strength at the object are well known in the art. For example, U.S. Pat. No. 5,391,199, and PCT patent application publication WO 96/05768, which are incorporated herein by reference, describe such systems for determining the coordinates of a medical probe or catheter inside the body. These systems typically include one or more coils within the probe, generally adjacent to the distal end thereof, connected by wires to signal processing circuitry coupled to the proximal end of the probe.
U.S. Pat. No. 4,710,708, which is incorporated herein by reference, describes a location determining system using a single axis solenoid with a ferromagnetic core as a radiating coil. There are a plurality of magnetic coil receivers. The position of the solenoid is determined assuming that it radiates as a dipole.
PCT patent application publication WO 94/04938, which is incorporated herein by reference, describes a position-finding system using a single sensing coil and an array of three, three-coil radiators. The radiator coils are wound on non-ferromagnetic forms. The position of the sensing coil is determined based on a dipole approximation to the magnetic fields of the coils where an estimate of the orientation of the sensor coil is first utilized in order to determine the position of the sensor coil in that order. Additionally, the radiator coils of each array are energized sequentially using a time multiplexing approach. Interestingly, although this reference discloses that frequency multiplexing can be utilized in order to significantly increase the operating speed of the position system, it clearly indicates that there are disadvantages to this type of approach due to its complexity. It is also important to note that although this reference teaches a single axis sensor position and orientation tracking system, it does not address any specific method for calibrating the system.
Accordingly, to date, there is no known system or method that provides for a electromagnetic position sensor single axis system and method that is capable of being simultaneously driven through frequency multiplexing utilizing a novel exact solution technique and a novel calibration method.
The present invention is a novel system and method used for determining the position and orientation of a medical device having a single sensor arranged along the longitudinal axis of the device. The system comprises a plurality of field radiators wherein each field radiator has a plurality of radiator elements. Each radiator element generates a magnetic field that is distinct from the others through its frequency which is sometimes referred to as xe2x80x9cfrequency multiplexingxe2x80x9d. A signal processor is operatively connected to the field radiators and the sensor of the medical device for receiving a sensing signal from the sensor indicative of the magnetic field sensed at the sensor. The sensing signal defines a measured magnetic field at the sensor. The signal processor also has a desired range of accuracy for the system which is stored therein. The signal processor includes an initial position estimator for establishing an initial position estimate for the sensor (which is based on the dipole approximation); a magnetic field calculator for calculating the magnetic field at the initial position estimate; a steepest descent calculator for calculating a steepest descent of the calculated magnetic field to the measured magnetic field; and a new position estimate calculator for calculating a new position estimate of the sensor based on the steepest descent. The magnetic field estimator and the steepest descent calculator use the exact theoretical field expressions and pre-stored calibration data which are unique to the system. The signal processor determines the position of the sensor when the new position estimate of the sensor is within the desired range of accuracy for the system.
The system also includes pre-stored calibration information for use with the magnetic field calculator and the steepest descent calculator for calculation of the magnetic field and the steepest descent step respectively. This calibration data is uniquely derived for each system using a novel calibration system and method along with its unique algorithm. The system also has a predetermined and stored desired range of accuracy of  less than 0.1 cm (the accuracy of the system). However, the incremental steps (iterations) for the algorithm are stopped as soon as the change from a previous step is less than 0.001 cm which is necessary in order to get better than 1 mm accuracy for the system.
One embodiment for the plurality of field radiators are arranged in a fixed arrangement and are contained in a fixed plane on a location pad. Other field radiator embodiments as described later do not necessarily have to lie in the same plane. In the first embodiment, the radiator elements of the field radiators are mutually orthogonal. In this embodiment, the system has three fixed radiators wherein each radiator has three generator elements or coils mutually orthogonal to each other.
Additionally, the signal processor determines both the position and orientation of the sensor such that the position of the sensor is derived in three different directions (X, Y, Z) and at least two orientations (pitch and yaw) which is generally known as 5 degrees of freedom (DOF). However, the restriction to 5 DOF is due to the coil sensor symmetry as shown. Thus, it is contemplated by the present invention to also provide for 6 DOF (X, Y, Z directions and three orientations roll, pitch and yaw) by changing the configuration of the sensor coil to an asymmetrical shape.
The system further comprises a display operatively connected to the signal processor for displaying the position and orientation of the sensor. Moreover, the display displays the position and the orientation of the sensor with respect to an anatomical feature of a patient. This is particularly useful for navigating a surgical instrument within a patient""s anatomy for performing a surgical procedure. The system further utilizes a reference device, which can be an external removable patch, for establishing a frame of reference. One particular use of the system is to map the heart thereby creating a 3D model of the heart. The sensor can be used together with a physiological sensor, such as an electrode in order to map a physiological condition, for instance, a local activation time (LAT).
The present invention also includes a novel method of determining the position and orientation of a sensor relative to a plurality of field radiators of known location wherein each of the field radiators comprises a plurality of co-located radiator elements. Each radiator element produces a differentiable field from all other field generating elements through frequency multiplexing. The sensor produces sensing signals indicative of the magnetic field at the sensor and from which the field at said sensor may be calculated. The method comprises the steps of:
(a) establishing a desired range of accuracy;
(b) determining an initial estimate of sensor position and orientation;
(c) calculating the magnetic field at the estimated sensor position and orientation;
(d) calculating the steepest descent from the calculated magnetic field at the estimated sensor position and orientation to the measured field at the sensor;
(e) calculating a new estimate for said sensor position and orientation from the steepest descent;
(f) iterating steps (c)-(e) based on the newly calculated sensor position and orientation estimate of step (e) to refine the sensor position and orientation estimate.
As mentioned above, the desired range of accuracy for the single axis sensor position and orientation system is xe2x89xa60.1 cm (the accuracy of the system). However, the incremental steps of the position and orientation algorithm are stopped as soon as the change from a previous step is less than 0.001 cm which is necessary in order to get better than 1 mm accuracy for the system. Additionally, the method includes establishing, storing and using of calibration information for the field radiators. This calibration information is derived using a novel calibration system and method. The calibration information is used at steps (c) and (d) for calculating a new estimate for the sensor position and orientation in order to provide greater accuracy to the system. The method also includes an optional step of refining the initial starting point of the sensor position and orientation using a dipole approximation in step (b).
The method further includes determining the position of the sensor in three different directions (X,Y,Z) and the orientation of the sensor in at least two orientations (pitch and yaw). Additionally, a display is used with this method for displaying the position and orientation of the sensor to include mapping this information to a displayed an anatomical feature of a patient which can be in the form of a pre-acquired image, real time image or model of the anatomy of interest.
The present invention also includes a novel calibration method which accounts for the effects of stationary metallic objects that are located within the mapping volume when the position and orientation medical system is in use. The novel calibration method is used for any medical system capable of generating a magnetic field for tracking a position of a medical device. The method comprising the steps of:
(a) defining a mapping volume within the generated magnetic field;
(b) placing a metallic object within the mapping volume;
(c) aligning a sensor at a first point within the mapping volume and measuring the magnetic field at the first point with the sensor to establish a first coordinate position (Xi, Yi, Zi);
(d) moving the sensor to a next point (Xi+dx, Yi+dy, Zi+dz) along one coordinate axis by an added distance component (dx, dy, dz) and measuring the magnetic field at the next point to establish a next coordinate position;
(e) interpolating the magnetic field at an intermediate point between the first position and the next coordinate position to establish an interpolated intermediate coordinate position;
(f) determining the position difference between the interpolated intermediate coordinate position and an actual intermediate coordinate position;
(g) comparing the position difference to an error limit;
(h) setting (Xi, Yi, Zi) of the next point as (Xi=Xi+dx, Yi=Yi+dy, Zi=Zi+dz) if the position difference is within the error limit and repeating steps (d)-(g) along another coordinate axis; and
(i) setting the added distance component (dx, dy, dz) by decreasing the value of the added distance component if the position difference is not within the error limit and repeating steps (d)-(g) along the same coordinate axis.
The method also includes completing the calibration method for the entire mapping volume in accordance with the steps outlined above. Although, the error limit can be any reasonable error range, it is preferable that the error limit be xe2x89xa61 mm for the greatest accuracy effects. Additionally, the sensor is stepped or moved a distance ranging from about 2 cm to about 3 cm. Moreover, with respect to the stepping of the sensor, the distance moved should remain constant to eliminate variability in the calibration. Also, step (i) is accomplished by decreasing the value of the added distance component through division by a factor of two (Xi+dx/2, Yi+dy/2, Zi+dz/2).
A second embodiment of the calibration method accounting for static metallic objects comprises the steps of:
(a) defining a mapping volume within the generated magnetic field;
(b) placing a metallic object within the mapping volume;
(c) aligning a sensor at a first point within the mapping volume and measuring the magnetic field at the first point with the sensor to establish a first coordinate position (Xi, Yi, Zi);
(d) extrapolating the magnetic field of a next point (Xi+dx, Yi+dy, Zi+dz) along one coordinate axis by an added distance component (dx, dy, dz);
(e) calculating the coordinate position at the extrapolated next point based on the extrapolated magnetic field to establish an extrapolated coordinate position;
(f) determining the position difference between the extrapolated coordinate position and the actual coordinate position of the next point;
(g) comparing the position difference to an error limit;
(h) setting the added distance component (dx, dy, dz) according to a predetermined distance if the position difference is within the error limit, aligning the sensor to a new point within the mapping volume along another coordinate axis and measuring the magnetic field at the new point with the sensor to establish a new point coordinate position and repeating steps (d)-(g) along the other coordinate axis; and
(i) setting the added distance component (dx, dy, dz) by decreasing the value of the added distance component if the position difference is not within the error limit and establishing an intermediate point by repeating steps (d)-(g) along the same coordinate axis.
The predetermined distance may remain constant and is preferably approximately 3 cm. However, the predetermined distance or step distance can be varied by the user as well. Additionally, the added distance component can be decreased by a factor of two such that the intermediate point or position is defined as (Xi+dx/2, Yi+dy/2, Zi+dz/2).
For either calibration embodiment accounting for the effects of stationary metallic objects, the sensor is moved according to the vertices of a cube and the entire mapping volume comprises a plurality of cubes. Each cube is defined by measurements derived from at least four different vertices. Generally, the calibration method is accomplished for a mapping volume is approximately 20 cmxc3x9720 cmxc3x9720 cm or (20 cm)3. For controlled accuracy in the calibration, the sensor is moved by the arm of a robot.